For about five years, Jack Greynolds Jr. can remember watching C.J. McCollum — all arms and legs — firing up jumpers. The GlenOak High School boys basketball coach knew McCollum had talent when he was in middle school. He knew McCollum could play by watching him in the summer.

Greynolds, though, never knew if McCollum would grow into the green GlenOak practice jerseys that seemed to hang on McCollum like curtains.

Fittingly, Thursday night McCollum grew into the NBA’s green room.

The 6-foot-3 Lehigh University graduate was selected with the 10th pick by Portland during Thursday night’s NBA Draft. It is one of McCollum’s best scenarios because the Trailblazers have guard Damian Lillard, the Rookie of the Year this past season. McCollum’s selection gives Portland one of the best young backcourts in the league.

“Never, ever had any inclination this would happen,” Greynolds said. “I mean, I knew C.J. had talent and he worked hard. But he was always young for his grade. I’m a firm believer if a young man is going to graduate high school when he’s 17, for maturity’s sake, you should hold him back. ... If C.J. had held himself back and played another year with us, he would have been off-the-charts good. Look at what happened to him between his senior year at GlenOak and his freshman year at Lehigh.”

McCollum grew.

He got stronger.

He grew some more.

Then one night two years ago in the NCAA Tournament, McCollum dropped 30 in one of the biggest NCAA Tournament upsets in history. He was so good, so poised that night that Duke head coach Mike Krzyzewski called McCollum the best player on the court.

McCollum probably would not have been in the green room if not for that night. Here’s why.

After that game, McCollum became a household name among elite college basketball players. He dipped his toes into the NBA’s waters after his junior season and came back.

But the Duke game opened doors for McCollum that would not have otherwise been opened. He was invited to Chris Paul’s camp in the summer. Then LeBron James’. Then Kevin Durant’s.

“And he did a great job at all three camps,” Greynolds said. “I think the Duke game put him on the map to get in those camps. His performance over last summer in those camps put him in the green room.”

Pairing McCollum with Lillard is a scenario McCollum didn’t want to count on because of the many draft-day scenarios. However, the two grew close after McCollum’s injury. Lillard injured a similar bone in the opposite foot as McCollum, and McCollum has talked with Lillard and received advice on life in the NBA.

How much does Portland like McCollum? Earlier this month he flew to Portland for a workout and was greeted by fans at the airport.

Everyone knows that McCollum’s senior season at Lehigh seemed doomed when he broke a bone in his left foot. He was the country’s leading scorer and he never played for Lehigh again after that injury in January.

“It made me better,” McCollum said.

He worked on shooting from a chair. He worked on his balance. He hit 3-pointers — 13 in a row at one point — standing on one foot. He worked out more. He got stronger.

Maybe most importantly, McCollum went back and broke down every game he played in. He knew the answers to the questions NBA coaches and general manager threw his way about particular games because he replayed them over and over in the film room.

Because of his size, though, McCollum almost became a baseball player. He was pretty good in middle school. But grass allergies — and the boredom of baseball — soon took over. He became a gym rat. The only person who believed, four years ago, and even four months ago, that McCollum would be in the green room for the NBA Draft was McCollum.

Greynolds used to joke with McCollum in front of the GlenOak team that if he ever grew, he could make money playing basketball one day.

“I still never thought he’d be a lottery pick,” Greynolds said. “I wasn’t saying he was going to play in the NBA when I said that. I figured maybe — maybe — overseas.”

Nope.

Just across the country, in Portland. Where they already know the kid, who’s grown into a man.